In a power distribution system, electrical transmission lines and power generation equipment must be protected against faults and short circuit events. Otherwise, such faults and short circuits can cause a collapse of the system, equipment damage, and/or personal injury. Accordingly, a typical power system employs one or more protective relays to monitor impedance and other AC voltage and current characteristics on a protected transmission line, so as to sense faults and short circuits on such protected line, and to appropriately isolate such faults and short circuits from the remainder of the power system by tripping pre-positioned circuit breakers on the protected line or lines.
In order to perform their task the relays receive current measurements performed on secondary circuits separated from the protected line by a current transformer (CT). Current transformers form the basic interconnection between the actual power system and almost all the measuring devices such as the protective relays. A CT steps the primary current down to a secondary level for use by the measuring device.
Due to their essential nature as devices with a primary and secondary winding coupled by the magnetic flux in a saturable iron core, CTs have known problems reproducing the primary current during fault events outside their designated specifications. As discussed for example in R. Hunt, “Impact of CT errors on protective relays—case studies and analyses,” IEEE Transactions Industry Applications, vol. 48, no. 1, pp. 52-61, 2012, the major sources of these problems are typically recognized as saturation effects, DC effects and remanent magnetic flux. To avoid errors in the reproduction of the secondary current, a CT is typically designed according to carefully drafted specifications taking into account the typical errors and their estimated magnitudes. Alternatively, or in addition, the saturation effects can be compensated for through the use of compensation algorithms as known in the art.
As described for example in the U.S. Pat. No. 6,397,156, it is further known that in uncompensated and compensated power systems a fault current waveform will contain an exponentially decaying DC offset component in addition to a fundamental frequency. The amount of the DC offset current is dependent on the fault inception angle and system parameters such as network configuration, number and length of transmission lines, compensation percentage, power flow, generator and transformer impedances, etc. In CT measurements the DC offset current is typically seen as an error and a variety of algorithms have been devised to compensate for the DC offset in current transformers. Some algorithms use a differentiation technique that eliminates the effect of the DC offset and ramp components in the fault current waveform. Mimic circuits and cosine filters have also been employed.
In SEAPAC 2011 34 (or 59) p. 1-18, “Test and Evaluation of Non Conventional Instrument Transformers and Sampled Value Process Bus on Powerlink's Transmission Network” by P. Schaub et al., a commercially available fiber optical current sensor (FOCS) is described linked together with a CT to a power line for test and evaluation purposes. A paper published in CIGRE 2012 B3-101 “Real-time Monitoring and Capture of Power System Transients” by D. F. Peelo et al. discusses transient measurement using optical voltage and current transformers.
US 2006/170410 A1 discloses a calibration method for a fiber-optic Faraday-effect current sensor (FOCS) in a power network. This calibration method makes use of a DC component present in the current measurement signal. For calibrating the sensor, the DC component of the measured AC value is compared to a stored DC signal value which was measured early during calibration of the system, and the percentage change from the calibrated DC component is multiplied to the AC component. Furthermore, the measured DC signal component is extracted from the FOCS measurement signal and is then subtracted and thus removed from the FOCS measurement signal. Therefore, the calibrated FOCS sensor signal is AC only and cannot be used to measure a DC signal component.
Joe-Air Jiang et al., “A Fault Detection and Faulted-Phase Selection Approach for Transmission Lines with Haar Wavelet Transform”, 2003 IEEE PES Transmission and Distribution Conference, Dallas, Tex., Sep. 7-12, 2003, p. 285-289 discloses a power line protection algo-rithm using the abrupt direct offset (DC) component of the current as a fault detection indicator. Simulated systems are discussed.
The invention starts from H. K. Karegar et al., “A new method for fault detection during power swing in distance protection”, Electrical Engineering/Electro-nics, Computer, Telecommunications and Information Tech-nology, 2009, ECTI-CON 2009, 6U International Conference IEEE, Piscataway, N.J., USA, 6 May 2009, p. 230-233. This article discloses a power line protection algorithm based on the DC component of fault currents being used for de-tecting a fault during power swing blockage of distance relays. Each phase current is measured and analyzed, the DC components are obtained and compared to thresholds, and if the DC components are bigger than the thresholds, the fault will be detected.
F. Maamoon al Kabajie et al., “A fault detec-tion and classification using new distance relay”, Renewble Energies and Vehicular Technology (Revet), 2012, First International Conference IEEE, 26 Mar. 2012, p. 237-243 discloses a power line protection algorithm using the abrupt direct offset (DC) component of the current as a fault detection indicator. Simulated and practical results are discussed.
U.S. Pat. No. 5,134,362 A discloses an apparatus for de-tecting failure in AC power systems using a first and second Faraday device whose output is processed by an electric circuit comprising a band-pass filter and a DC filter. The DC components correspond to cases with no magnetic field or magnetic fields of same direction being applied to the Faraday devices, whereas the AC components correspond to a magnetic field applied to either Faraday device or to magnetic fields applied to both Faraday devices in opposite directions. The output of the DC filter is used to normalize the output of the band-pass filter to eliminate errors due to differences in light attenuation in the optical fibers.